1. Field of the Invention
The present invention concerns an improvement in analog-digital converters, called "flash" analog-digital converters. This improvement consists in introducing a logic correction stage between the stage of the comparators and the encoder stage. This correction stage forces the output of an accidentally erroneous comparator to assume the same logic state, namely 0 or 1, as the outputs of the two neighboring comparators if these outputs are identical.
In other words, the correction circuit according to the invention sets the output of a comparator at 0, if this comparator output gives a logic 1 while the two comparators on either side of it both give a logic 0. Reciprocally, it sets the output of a comparator, giving a logic 0, at logic 1 if the two comparators on either side of it give a logic 1.
2. Description of the Prior Art
It is known that a flash-type analog-digital converter (other ADC structures exist) comprises, as very briefly recalled in FIG. 1, a first stage of parallel-mounted comparators 1,2. . . i,i+1, . . . 2.sup.n -2; 2.sup.n -1, if the ADC has to give a logic signal formed by n bits. The analog signal to be converted, at the input E, is applied in parallel to all the first inputs of the comparators. Reference voltages, in equal discrete values, are tapped at a voltage divider between V.sub.min and V.sub.max, and are applied to the two inputs of the comparators.
The set forming the comparators and the voltage divider constitutes the comparator stage, referenced 3, which gives what is commonly called a "thermometer scale". This is to say that all the outputs of comparators for which the analog voltage is greater than the reference voltage give a logic 1 signal Reciprocally, all the outputs of the comparators for which the analog voltage is smaller than the reference voltage give a logic 0 signal.
The set of these (2.sup.n -1) logic values, namely a series of logic "ones" followed by a series of logic "zeros", or thermometer scale, is addressed to an encoding stage 4 that gives a binary number of "n" bits.
In fact, and above all in the field of microwaves with an analog frequency oscillating between 1 and 100 GHz, it can happen that the comparators do not work perfectly, and that one of them accidentally gives a logic 1 instead of a logic 0, or vice versa. Such accidents are due, for example, to a stray phenomenon during only one measurement (the comparators are all driven by a clock), or again because a comparator is isolated during calibration and is, therefore, temporarily incapable of working.